1. Field of the Invention
The present invention concerns an aluminum alloy material having a surface of excellent zinc phosphate processability and it relates to a surface treated aluminum alloy material suitable to such an application use that coating is applied after pretreatment with zinc phosphate processing, in particular, to automobile panel materials.
2. Description of the Prior Art
Aluminum alloys have been employed in recent years to automobile parts with an aim of reducing weight and a pretreatment for coating such as with chronic chromate has been necessary in such an application use requiring filmform corrosion resistance such as in panel materials.
Although zinc phosphate processing has been employed for the pretreatment in usual automobile coating lines, no sufficient effect can yet be obtained at present as the pretreatment for aluminum alloys in view of the filmform corrosion resistance.
As the aluminum alloy materials for automobile panels, Al--Mg--Cu type alloys have been used predominantly, because the aluminum alloys of this type tend to readily cause deposition of zinc phosphate and are excellent in the filmform corrosion resistance as compared with Al--Si--Mg type alloys.
Although the Al--Si--Mg type alloys have high strength after baking of the coating and have excellent properties as the automobile panel material, since they cause less deposition of zinc phosphate and can not provide sufficient filmform corrosion resistance as described above, they have been scarcely used at present for the application use of automobile panels using the zinc phosphate processing as the pretreatment.
On the other hand, an attempt of improving the zinc phosphate processability by applying Zn plating to the surface of an aluminum alloy plate has already been conducted, for example, in Japanese Patent Laid-Open Sho 61-157693. Such a technique has an aim of improving the zinc phosphate processability and preventing dissolution of aluminum ions into a zinc phosphate bath by means of a Zn plating layer. Accordingly, about 1 g/m.sup.2 of amount is required as coating weight of plating and Zn plating has to be applied as far as inner panels not requiring the coating surface finishing property or the filmform corrosion resistance. However, if the processing area or the average amount per unit area is increased, the processing cost is increased which is not desirable industrially. Further, if the Zn layer is left after the zinc phosphate processing, it tends to cause swelling in the coating layer and hence is not preferred.
Further, in the prior art, although the filmform corrosion resistance can be improved, for example, in a case of the chronic chromate treatment, the following steps are necessary, and exclusive processing facilities are required, as well as there is a problem that a processing cost including that for waste water disposition is increased.